Apparatus and method of limiting power applied to a loudspeaker

ABSTRACT

A method of limiting the power applied to a loudspeaker is disclosed, in which both the voltage and current applied to the loudspeaker are measured, and instantaneous power is directly calculated and used to control the level of the input signal that drives the amplifier powering the loudspeaker. When the power applied to the loudspeaker exceeds a prescribed threshold, the input level to the power amplifier is reduced until the measured power falls below the threshold. Also disclosed is a method for indirectly determining the voice coil temperature from the loudspeaker&#39;s voltage and current and reducing power to the loudspeaker when the temperature exceeds a prescribed threshold. The power level is actively controlled to prevent damage to the loudspeaker and to minimize audibly objectionable artifacts.

REFERENCE TO RELATED APPLICATION

[0001] This application is related to and claims priority from U.S.provisional patent application Serial No. 60/454,271, filed on Mar. 12,2003, which is hereby fully incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to power regulation and, inparticular, to the regulation of power applied to loudspeakers.

[0004] 2. Background

[0005] In many applications, it is desirable to drive a loudspeaker asloudly as possible, without causing audible distortion or damage to theloudspeaker. Loudspeakers convert electrical energy into acoustic energyand thermal energy. When alternating electrical energy (power) isapplied to the leads of a loudspeaker voice coil, forces are createdwhich interact with the magnetic field in a magnetic gap. For example,in the conventional loudspeaker 10 of FIG. 1, the voice coil 12, whichis attached to the coil form 14, will move in and out of the magneticgap 16 in response to the alternating power being applied. Thisinteraction may result in cone motion (a.k.a. excursion). Because thevoice coil is rigidly connected to the cone 18, the cone 18 and theloudspeaker spider 20 will move with the same motion as the voice coil12, thereby producing sound. The voice coil 12, as shown in FIG. 1, issituated between a front plate 22 and a pole piece 24 in the magneticgap 16. In the typical loudspeaker design, the magnet 26 is held intoplace between front plate 22 and a back plate 28. Moreover, the venthole 30 allows heat to dissipate, while the dust dome 32 protects thevoice coil area from potentially harmful debris.

[0006] Virtually all loudspeaker damage results from the application ofexcessive power to the loudspeaker. Loudspeakers are highly inefficientand convert most of their applied power into heat. A significantcomponent of this heat, which is a function of wasted power over time,is generated in the voice coil 12. This heat is transferred to thesurrounding parts of the loudspeaker, mostly through conduction andconvection.

[0007] Types of loudspeaker damage include thermal degradation, thermalfailure, mechanical failure or a combination thereof. Thermaldegradation results from the repeated application of excessive powerover a period of time. Repeated overheating of the voice coil 12 leadsto damaging thermal-expansion effects and material fatiguing. Suchthermal degradation can further lead to thermally-induced mechanicalfailure since the materials that comprise the voice coil 12 and relatedcomponents tend to become brittle and are generally more vulnerable tomechanical shock. In addition, heating of the voice coil will causeheating of the surrounding materials; overheating of the magnet 26 canirreversibly change its magnetic properties, especially magnets made ofrare-earth materials such as neodymium.

[0008] Thermal failure, like thermal degradation, results from thenegative heat effects of applying excessive power to a loudspeaker.However, instead of causing gradual degradation, a strong power surgecan lead to a catastrophic thermal failure in which the voice coil 12 isheated to the point where it or other components of the loudspeakerliterally melt, break or bum.

[0009] On the other hand, mechanical failure may occur when excessivepower moves the voice coil 12 far enough that it strikes the back plate28 or separates from the coil form 14. Similarly, the application ofexcessive power can cause the voice coil 12 to put excessive stress onthe cone 18 or spider 20, causing tearing. In any of these cases, thevoice coil 12 may become misaligned since the cone assembly is notsuspended properly. It is this voice coil misalignment or cone/spidertearing that will lead to mechanical failure.

[0010] Existing methods of power limiting include measuring the voltageapplied to the loudspeaker and limiting the power based on assumptionsregarding loudspeaker impedance. However, such methods do noteffectively limit the power delivered to the loudspeaker.

[0011] In U.S. Pat. No. 4,233,566, Nestorovic discloses a method oflimiting power to a loudspeaker based on the assumption that theloudspeaker is a fixed resistive load. However, loudspeaker impedance isnot simply a resistive load, but rather varies with frequency and drivertemperature. Therefore, the power delivered to the loudspeaker cannot beaccurately determined by assuming the loudspeaker is a fixed resistiveload.

[0012] In U.S. Pat. No. 4,327,250, von Recklinghausen describes a methodof limiting that uses a model of the loudspeaker. The voltage presentacross the loudspeaker terminals drives the loudspeaker model, and theoutput of the model is compared to a threshold. However, limiting isbased solely on the output of the loudspeaker model, which is merely anestimation and not a measurement of power or voice coil temperature.

[0013] In U.S. Pat. No. 4,216,517, Takahashi describes a method ofcircuit protection for a power amplifier in which both voltage andcurrent are detected. This method attempts to protect the amplifier fromdamage when the load impedance is too small. It does not protect theloudspeaker from excessive power or distortion.

[0014] In U.S. Pat. No. 5,719,526, Fink describes a method in which boththe voltage and current applied to a loudspeaker are measured, and poweris calculated as a function of frequency. A record of measurement datais stored and a Fourier transformation performed, where the minimumlength of this record is dictated by the lowest frequency of interest inthe signal. For audio, this lowest frequency is 20 Hz, or a minimumrecord length of 50 milliseconds. The length of this record severelylimits the response time of processing that may occur on the inputsignal as a result of the measured power output.

[0015] In U.S. patent application Ser. No. 2002/0118841, Buttondescribes a system that limits the temperature of a loudspeaker's voicecoil by estimating the voice coil temperature and comparing it to apredetermined threshold. This system uses the input signal and a thermalmodel of the voice coil to estimate the voice coil temperature. However,a thermal model is inherently susceptible to inaccuracy, because it isan estimation of the voice coil temperature and not an actualmeasurement.

[0016] Accordingly, there is a need in the technology for apparatus andmethods that overcome the aforementioned problems.

SUMMARY OF THE INVENTION

[0017] Methods and apparatus for limiting power to a load are disclosed.One aspect of the invention involves a method that comprises driving theload with an input signal from a power source, providing a power signalthat is representative of a power level of the input signal andcalculating a value based on said power signal according to one or morecontrol parameters. The method further comprises limiting the inputsignal based on the value.

[0018] A second aspect of the invention involves mitigation or reductionof degradation of loudspeakers due to thermal effects.

[0019] Other embodiments are disclosed and claimed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a simple schematic of a loudspeaker;

[0021]FIG. 2 is a simplified diagram of one embodiment of a powerlimiter, in accordance with the principles of the present invention;

[0022]FIG. 3 is a diagram of a second embodiment of the presentinvention;

[0023]FIG. 4 is a diagram of an embodiment of the control element ofFIG. 3;

[0024]FIG. 5 is a diagram illustrating one embodiment of the attenuationelement of FIG. 3, where attenuation is performed digitally and theattenuated signal is converted to analog using a digital-to-analogconverter;

[0025]FIG. 6 is a diagram illustrating one embodiment of the attenuationelement of FIG. 3, where attenuation is performed digitally and theattenuated element output is a digital signal;

[0026]FIG. 7 is a diagram illustrating one embodiment of the attenuationelement of FIG. 3, where attenuation is performed using a digitallycontrolled analog signal;

[0027]FIG. 8 is one embodiment of a process for limiting powerconsistent with the principles of the invention;

[0028]FIG. 9 is one embodiment of a process for calculating an updatedgain value to be used in the process of FIG. 8;

[0029]FIG. 10 is an illustration of the effect of temperature onloudspeaker impedance versus frequency; and

[0030]FIG. 11 is a diagram of an embodiment of the control element ofFIG. 3 for performing thermal limiting.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] The present invention relates to an apparatus and method oflimiting the power delivered to a loudspeaker in order to preventdistortion or damage caused by signals of excessive magnitude. In oneembodiment, the technique is able to quickly limit the power whileminimizing the audible artifacts of the limiting process itself.

[0032] One aspect of the invention relates to limiting the true powerdelivered to a loudspeaker by measuring both voltage and current. Thepower is actively controlled in response to the measured voltage andcurrent. In one embodiment, the invention relates to limiting the powerdelivered to a loudspeaker with no prior knowledge of the loudspeaker'scharacteristics.

[0033] A second aspect of the invention relates to an apparatus andmethod that quickly limits the power applied to a loudspeaker beforedamage or audible distortion can occur; and in doing so, the limitingprocess should not generate its own audibly objectionable distortion.

[0034] A third aspect of the invention provides the user withindependent parameters with which to control the power limiting process.These parameters include but are not limited to (1) power levelaveraging time, (2) power threshold, (3) attack time, and (4) releasetime. In one embodiment, the invention allows the user to select betweenpeak power limiting and average power limiting.

[0035] A further aspect of the invention involves limiting thetemperature of the voice coil of the loudspeaker before damage to theloudspeaker can occur. The temperature of the voice coil, which isdetermined by its DC resistance, may be directly calculated from thevoltage and current measured at the loudspeaker. In another embodiment,only the loudspeaker's impedance function needs to be known, and thetemperature of the voice coil can be determined given any input signalof sufficient magnitude.

[0036] In one embodiment of the invention, both the voltage and currentapplied to the loudspeaker are measured. Using both measurements,instantaneous power can be directly calculated as the product of the twoand used to control the level of the input to the amplifier in afeedback configuration. In another embodiment, the temperature of thevoice coil is calculated from the measured voltage and current and usedto control the level of the input to the amplifier in a feedbackconfiguration. The level of the input may also be controlled quicklyenough to prevent damage to the loudspeaker and precisely enough tominimize audibly objectionable artifacts.

[0037]FIG. 2 illustrates a simplified version of the present invention.Input signal 34 is provided to attenuator 36, the output 38 of which isprovided to power amplifier 40, which drives loudspeaker 46. Voltagemonitor circuit 48 generates signal 50 that is representative of thevoltage present at loudspeaker input 44. Current monitor circuit 42generates signal 52 that is representative of the current flowingthrough loudspeaker 46. Signals 50 and 52 drive control circuit 58. Thecontrol circuit generates gain value 60 that is used to attenuate inputsignal 34 when loudspeaker 46 is in danger of being damaged or producingdistortion.

[0038] One embodiment of the invention is shown in FIG. 3. The level ofinput signal 34 is adjusted by attenuator 62, whose output 38 drivespower amplifier 40. The output of power amplifier 40 passes throughcurrent monitor circuit 64 and drives loudspeaker 46.

[0039] Loudspeaker 46 in this embodiment is representative of a singleloudspeaker or a plurality of loudspeakers connected by electricalmeans. The loudspeaker may or may not be connected to a passivecrossover network. Current and voltage monitoring may occur before orafter this crossover network, if available.

[0040] Current monitor circuit 64 generates a voltage that isrepresentative of the current flowing through loudspeaker 46. Thiscurrent may be measured as the current that flows from the output ofpower amplifier 40 to loudspeaker input 44. Analog-to-digital converter66 samples the voltage signal generated by current monitor circuit 64.Voltage monitor circuit 68 generates a voltage that is representative ofthe voltage present at loudspeaker input 44. Analog-to-digital converter70 samples the voltage signal generated by voltage monitor circuit 68.

[0041] The particular implementations of monitoring circuits 64 and 68are not covered by this invention, and it is assumed that their effecton loudspeaker input signal 44 is negligible. FIG. 3 shows thesemonitoring circuits as separate from amplifier 40, but it should equallybe appreciated that they may be part of amplifier 40. In such anembodiment, the outputs of these monitoring circuits accuratelyrepresent the voltage and current at the output of amplifier 40. Inanother embodiment, the electrical connection between the output ofamplifier 40 and loudspeaker 46 may have only a negligible effect on thevoltage and current at loudspeaker input 44.

[0042] Monitoring circuits 64 and 68 scale the magnitudes of thevoltages they generate so that analog-to-digital converters 66 and 70may sample them. Any DC offset that is present may be removed fromoutputs 52 and 50 of analog-to-digital converters 66 and 70,respectively.

[0043] In one embodiment, analog-to-digital converters 66 and 70 sampletheir inputs at greater than or equal to the Nyquist rate, or at a ratethat is twice the highest frequency present in output signal 44.However, the scope of this invention is not limited to sampling at theNyquist rate. If the voltage and current monitor signals are subsampled(sampled at less than the Nyquist rate), aliasing may occur. In thiscase, the instantaneous power is calculated as the product of thesubsampled voltage and the subsampled current. Subsampling does notalter the average power measurement of the sampled signal; therefore,limiting the average power output based on a subsampled instantaneouspower measurement is valid. It may be necessary to modulate the samplingfrequency in order to accurately detect frequencies that would otherwisebe an integer multiple of the sampling frequency. One embodiment willmodulate the sampling frequency randomly or pseudo-randomly betweenspecified lower and upper boundary frequencies.

[0044] Signals 50 and 52, which are representative of the voltage andcurrent present at the loudspeaker, are provided as inputs to controlelement 58. User-input control parameters 72 may be used to determinethe behavior of control element 58. In one embodiment, user-inputcontrol parameters 72 are provided as a library of user-input controlparameters 72. In another embodiment, the library may contain user-inputcontrol parameters 72 that have been optimized for particularloudspeakers, and these optimized control parameters 72 may be accessedby selecting a corresponding loudspeaker from a list of availableloudspeakers. In yet another embodiment, a user may manually set theuser-input control parameters 72. In a further embodiment, the libraryof control parameters or user-selected parameters may be accessedthrough a graphical user interface.

[0045] Gain value 60 may be provided as an input to attenuator 62, whichadjusts the level of input signal 34. In one embodiment, control element58 may be implemented as firmware, software or hardware. In anotherembodiment, the control element 58 may be implemented as a linear ornonlinear gain control element. It may also be implemented as arecursive or non-recursive gain control element.

[0046] One embodiment of control element 58 is shown in FIG. 4.Multiplier 54 calculates the instantaneous power measurement as theproduct of signals 50 and 52, which are representative of the voltageand current, respectively. The power level is calculated by averagingfilter 74 from the output of multiplier 54. In one embodiment, averagingfilter 74 is a second-order recursive filter. In this embodiment, twofirst-order low-pass filters are cascaded to produce the second-orderaveraging filter. Averaging coefficient 92, T_(A), is specified by theaveraging time: $T_{A} = ^{\frac{- n}{t_{a}f_{S}}}$

[0047] where n is the filter order, t_(a) is the averaging time inseconds, and f_(S) is the sampling frequency in Hz. The scope of thisinvention encompasses other types of level measurements including butnot limited to recursive averaging of any order and filter type,nonrecursive (transversal) averaging, and combination peak/averagemeasurements.

[0048] If the averaging time of averaging filter 74 is set to zero, ineffect bypassing the averaging filter, gain reduction may be calculatedbased on peak power. Setting the averaging time to greater than zeroresults in limiting of the average power over the time specified byaveraging coefficient 92. In this implementation, therefore, either peakpower or average power may be limited. In one embodiment, the averagingtime is greater than or equal to 50 milliseconds for average powerlimiting over the audio range of frequencies. The average power level(L), the output of averaging filter 74, is processed by gain calculationelement 76. In one embodiment, the gain calculation element 76calculates the gain such that the input signal is attenuated by anappropriate amount when the average power level (L) exceeds threshold94, P_(T). The output of gain calculation element 76 may be calculatedas follows: $\begin{matrix}{{gain} = \sqrt{\frac{P_{T}A_{I}A_{V}}{L}}} & (2)\end{matrix}$

[0049] where P_(T) is specified in Watts, A_(I) and A_(V) are correctionfactors for the respective gains of current monitor circuit 64 andvoltage monitor circuit 68, and L is the output of averaging filter 74.

[0050] The scope of this invention encompasses other types of control,including but not limited to linear or nonlinear gain control, andpiecewise linear or piecewise nonlinear gain control.

[0051] Multiplier 78 calculates the product of the output of gaincalculation element 76 and the output of averaging filter 90. The outputof multiplier 78 is provided to gain processing element 80. In oneembodiment, gain processing element 80 inverts the gain value such thatits output is a measure of gain reduction and constrains the value suchthat it is non-negative.

[0052] In one embodiment, peak detector 82 calculates the peak gainreduction from the output of gain processing element 80. The purpose ofthe peak filter is to maintain the gain reduction level over a userspecified release time. A peak filter is a recursive low-pass filterthat stores the maximum of its input and output as feedback to the nextsample cycle. The release time, t_(R), of the filter is determined byrelease coefficient 96, T_(R): $\begin{matrix}{T_{R} = ^{\frac{- n}{t_{R}f_{S}}}} & (3)\end{matrix}$

[0053] In one embodiment, the peak filter is third order (n=3), althoughany filter order is within the scope of this invention. The third-orderfilter may be constructed by cascading three first-order filtersections. Each filter section compares its input and output and storesthe maximum as feedback to the next sample cycle.

[0054] Typically, the output of peak detector 82 should not be allowedto change too rapidly, or its nonlinear effect on attenuated inputsignal 38 would be audible. Therefore, the output of peak detector 82 issmoothed by smoothing filter 84. In one embodiment, a second-orderBessel-Thomson filter may be used for smoothing filter 84, although anyorder or type of filter is within the scope of this invention. TheBessel-Thomson filter is used to prevent overshoot of the smoothed gainvalue during transition. A second-order filter was chosen because itadequately attenuates higher-order harmonics in the non-linear output ofpeak detector 82. If insufficiently attenuated, these higher-orderharmonics would modulate the input signal at attenuator 62, resulting inaudible aliasing.

[0055] In another embodiment, cascaded first-order filters may be usedfor smoothing filter 84. Although this results in a greater group delaythan the Bessel-Thomson filter, it may be simpler to implement and willnot result in overshoot of the smoothed gain value.

[0056] When averaging time 92 is equal to zero, smoothing filter 84determines the minimum time in which the onset of gain reduction canoccur. This time is specified as the attack time. The attack time is theamount of time required for gain value 60 to change by a specifiedamount, given that the output of gain calculation element 76instantaneously changes from no gain reduction to the steady statedestination value. The attack time may also be defined as the timeconstant of smoothing filter 84. The attack time determines smoothingcoefficient 98, T_(S).

[0057] In the described embodiment, the minimum release time is equal tothe attack time. In other words, the release time cannot be less thanthe attack time. This constraint is acceptable in the vast majority ofaudio applications, and this embodiment is preferred over one that usesa single switched filter that determines both attack and releasecharacteristics. A single switched filter would have a time constantthat is switched between attack and release times, depending on theslope of the input to this filter. This switched filter is inherentlynonlinear, and would produce aliasing in attenuator output 38. Theplacement of a linear smoothing filter at the end of the control signalpath as described by the present invention greatly reduces audiblealiasing.

[0058] Gain inversion element 86 converts the output of smoothing filter84, which is a measure of gain reduction, into gain value 60.

[0059] Gain value 60 is also provided back into multiplier 78 throughloop filter 88 and averaging filter 90. Averaging filter 90 is designedto mimic averaging filter 74, and therefore has the same averaging time92. In one embodiment, loop filter 88 is designed to mimic the effect ofthe remainder of the feedback path: gain value 60, which is applied tosignal input 34, which is amplified by amplifier 40, whose current andvoltage are monitored by circuits 64 and 68, whose outputs are sampledby analog-to-digital converters 66 and 70. In another embodiment, loopfilter 88 is a simple delay whose delay time is equal to that of thepreviously described feedback path.

[0060] In one embodiment, the sample rate of the control signal may bedecimated at any point after averaging filter 74. Such decimation mayreduce the amount of computation. In one embodiment of the above case,the control signal can be interpolated by smoothing filter 84, restoringthe sample rate to its previous value.

[0061] Gain value 60 is provided to attenuator 62, where it controls thelevel of the input to amplifier 40. One embodiment of attenuator 62 isshown in FIG. 5. Analog-to-digital converter 100 samples input signal34. Multiplier 102 calculates the product of the sampled input and gainvalue 60. The output of multiplier 102 is converted to analog output 38by digital-to-analog converter 104. Analog output 38 is amplified byamplifier 40 of FIG. 3.

[0062] A second embodiment of attenuator 62 is shown in FIG. 6.Analog-to-digital converter 100 samples input signal 34. Multiplier 102calculates the product of the sampled input and gain value 60. Digitaloutput 38 is modulated and amplified by amplifier 40 of FIG. 3. The typeof modulation in this embodiment may be pulse-width modulation,sigma-delta modulation, a variation of one of these techniques, or anyother type of digital modulation technique.

[0063] In both attenuator embodiments shown in FIG. 5 and FIG. 6, inputsignal 34 may be delivered in digital form using a digital transmissionmedium. In this case, it is assumed that analog-to-digital converter 100is present at some point in the signal chain before multiplier 102. Inthe case of a digitally generated signal, analog-to-digital converter100 may be omitted without altering the scope of this invention.

[0064] A third embodiment of attenuator 62 is shown in FIG. 8. Input 34is provided to digitally-controlled analog attenuator 108, which iscontrolled by gain value 60. Analog output 38 is amplified by amplifier40 of FIG. 3.

[0065] In all three of the above-mentioned embodiments, additionalsignal processing may occur before or after multiplier 102 or attenuator108.

[0066] In other embodiments, the invention may be extended to amulti-channel power limiter. Each channel consists of an input 34 thatis provided to attenuator 62, which feeds amplifier 40, which in turndrives loudspeaker 46. Current monitor circuit 64 generates a voltagerepresentative of the current that flows through loudspeaker 46, whichis sampled by analog-to-digital converter 66. Voltage monitor circuit 68generates a voltage representative of the voltage at loudspeaker input44, which is sampled by analog-to-digital converter 70. Digital currentsignal 52 and digital voltage signal 50 are provided to multiplier 54,which calculates the instantaneous power 56.

[0067] In one embodiment of a multi-channel power limiter, the maximuminstantaneous power of all the channels is chosen as the input tocontrol element 58. This control element is common to all channels.Other combinations of the channels, such as the maximum of all channels'average power, or the average of all channels' power measurements, arewithin the scope of this invention. The remainder of the control path isequivalent to that of FIG. 4. Resultant gain 60 is used on all channelsto prevent the virtual audio image from shifting.

[0068] Referring now to FIG. 8, in which one embodiment for limitingpower to a loudspeaker (e.g., loudspeaker 46) is depicted. Such aprocess may be implemented as software, firmware or hardware. Inparticular, process 200 begins with an input signal being provided to anattenuator at block 202. In one embodiment, the input signal is inputsignal 34, which is being provided to attenuator 62. The attenuator maythen regulate the level of the input signal according to a current gainvalue (block 204). Thereafter, at block 206, the attenuator may be usedto drive an amplifier (e.g., amplifier 40) with the attenuated inputsignal (e.g., output 38). The amplifier may then provide power to aconnected loudspeaker. At block 208, a current monitor and voltagemonitor may be used to measure the amplifier's current level and voltagelevel, respectively. In one embodiment, the current monitor is currentmonitor circuit 64 and the voltage monitor is voltage monitor circuit68.

[0069] Process 200 may then continue to block 210 where the currentmonitor and voltage monitor generate representative signals of thecurrent and voltage levels being provided to the loudspeaker. Thesesignals may then be used to calculate an updated gain value, which inone embodiment is calculated according to the process discussed belowwith reference to FIG. 9. The updated gain value, however calculated,may then be supplied back to attenuator block 204, which in turn, mayadjust the level of attenuation based on the updated gain signal.

[0070] Referring now to FIG. 9, in which one embodiment of a process 218for calculating the updated gain value for block 214 of FIG. 8. Theprocess 218 for calculating the updated gain value of block 214 beginsat blocks 220 and 222, when the respective voltage and currentmeasurements are received by a control block (e.g., control element 58).Process 218 then calculates the instantaneous power at block 224. Fromthis instantaneous power, process 218 calculates a signal representativeof the power level (block 226). In one embodiment, this representativesignal level is calculated by averaging filter 74. While in anotherembodiment the representative signal level is an averaged measurement ofthe power signal, such as one of the averaging measurements discussedabove with reference to FIG. 4, it should also be appreciated that therepresentative signal level may be a peak measurement. In yet anotherembodiment, averaging coefficient T_(A) may be used to calculate therepresentative signal level of block 226.

[0071] Thereafter, at block 228, the representative signal level andpredetermined power threshold are used to calculate a gain value. Thiscalculated gain value is representative of the additional gain requiredat block 204 (e.g., attenuator 62) to achieve an attenuated signal levelequal to the predetermined power threshold. In one embodiment, this userinput power threshold is P_(T). In one embodiment, the gain valuecalculation proceeds according to Equation 2.

[0072] Continuing to refer to FIG. 9, the gain value previouslycalculated at block 244 is supplied to block 230. This gain value isprocessed by a filter that mimics the response of the loop, or thesignal path between the output of block 214 and the input of block 214.In one embodiment, this filter is a pure delay. Process 218 may thencontinue to block 232 where the average value of the filtered gain valueis calculated. In one embodiment, this average is calculated in the samemanner as that of block 226.

[0073] Process 218 may then continue to block 234 where the additionalgain value calculated by block 228 is multiplied by the average of theprevious gain value supplied by block 232. The resulting value is thegain required at block 204 (e.g., attenuator 62) to achieve anattenuated signal level equal to the predetermined power threshold. Theoutput gain of block 234 is constrained and inverted at block 236, suchthat the output value is between 0 and 1 and is equal to 1 minus theinput value.

[0074] Continuing to refer to FIG. 9, block 238 calculates the peaklevel of the gain reduction, which maintains the gain reduction levelfor a specified period of time. In one embodiment, the specified periodof time is a function of release coefficient T_(P).

[0075] At block 240, the gain reduction value calculated at block 238may be smoothed using a smoothing filter, such as smoothing filter 84.Afterwards, the smoothed gain reduction value is re-inverted at block242 to yield a gain value. This inversion process may be equivalent tothat of block 234. Finally, at block 244 of process 218, the resultantsmoothed gain value may then be supplied to a signal attenuator (e.g.,attenuator 62) as an updated gain value.

[0076] Another aspect of the invention involves the reduction/preventionof thermal degradation of loudspeakers. Nearly all thermally inducedfailures of loudspeakers result from voice coil overheating. The changein DC resistance of a loudspeaker's voice coil, $\frac{R_{E}}{R_{0}},$

[0077] is dependent only upon its change in temperature and may be foundby the following expression: $\begin{matrix}{\frac{R_{E}}{R_{0}} = {1 + {\alpha \left( {T - T_{0}} \right)} + {\beta \left( {T - T_{0}} \right)}^{2}}} & (4)\end{matrix}$

[0078] where R_(E) is the DC resistance of the voice coil at operatingtemperature T, R₀ is the DC resistance of the voice coil at ambienttemperature T₀ (typically 25 Celsius), and α and β are the thermalcoefficients of resistance for the material from which the voice coil ismade (typically copper or aluminum). In addition, it is generallyaccepted that R_(E) is the only temperature-dependent component of theequivalent electrical circuit of a loudspeaker. This means that theloudspeaker's impedance as a function of frequency, Z(ω), will shiftupward with an increase in temperature, while its shape is generallyunchanged. (In this context, the term loudspeaker refers to aloudspeaker mounted in an enclosure.) An illustration of this change inloudspeaker impedance with temperature is shown in FIG. 10.

[0079] Dividing the loudspeaker's impedance at temperature T by itsimpedance at T₀ results in $\frac{R_{E}}{R_{0}},$

[0080] from which the increase in voice coil temperature may bedetermined. Since the voltage driving the loudspeaker is unchanged bythe loudspeaker's impedance, only the current flowing through theloudspeaker changes. Thus, only the change in current is needed todetermine the change in temperature: $\begin{matrix}{\frac{R_{E}}{R_{0}} = {\frac{Z\left( {\omega,T} \right)}{Z\left( {\omega,T_{0}} \right)} = {{\frac{V}{I\left( {\omega,T} \right)} \cdot \frac{I\left( {\omega,T_{0}} \right)}{V}} = \frac{I\left( {\omega,T_{0}} \right)}{I\left( {\omega,T} \right)}}}} & (5)\end{matrix}$

[0081] One exception to this is a phenomenon known as port compression,which results when a loudspeaker in a ported enclosure is driven at highsound pressure levels. As the drive level of this loudspeaker isincreased, the acoustic output at the port frequency increasesproportionally less; and at high enough drive levels, the acousticoutput actually decreases. This compression effect is evident in theloudspeaker's equivalent electrical impedance function and must becompensated for.

[0082] The current that flows through the loudspeaker is a function ofthe impedance of the loudspeaker and the voltage applied to it. In otherwords, this current may be modeled as the output of an admittancefunction, A(ω), where the voltage applied to the loudspeaker is theinput to A(ω).

[0083] A(ω) is a function of frequency and is the reciprocal of theloudspeaker's impedance function. For present purposes, A(ω) may bedetermined at ambient temperature T₀ such that A(ω) does not change withtemperature. With the measured voltage from the loudspeaker as input toA(ω), the temperature-invariant contribution to the current flowingthrough the loudspeaker may be estimated. Note that A(ω) may be afunction of the power applied to the loudspeaker, as is the case inloudspeaker port compression.

[0084] Dividing a temperature-invariant estimation of the current by themeasured current from the loudspeaker results in $\frac{R_{E}}{R_{0}},$

[0085] which is the only temperature-dependent portion of theloudspeaker's impedance. This estimate can be made for any signalapplied to the loudspeaker; therefore, the change in voice coilresistance under normal operating conditions can be measured. Theexception to this is when there is insufficient input level to make anaccurate measurement; however, under these conditions, the voice coil isnot susceptible to overheating. The level of the measured current andmodeled current may be averaged to minimize differences due todeficiencies in A(ω).

[0086] Given the maximum temperature of the voice coil (and assumingthat the actual ambient temperature is not significantly different thanT₀), the maximum change in resistance of the voice coil can becalculated from Equation (4). By comparing this threshold value with anestimated change in voice coil resistance, $\frac{R_{E}}{R_{0}},$

[0087] the point at which the applied power should be reduced can bedetermined. Reducing the power applied to the loudspeaker will lower thetemperature of the voice coil.

[0088] Since β in Equation (4) is typically small, the change in voicecoil resistance is nearly linearly related to the change in temperature.Therefore, the gain reduction calculation can be based on$\frac{R_{E}}{R_{0}},$

[0089] resulting in an appropriate change in voice coil temperature.

[0090]FIG. 11 shows control element 58, which has been adapted to limitthe power applied to loudspeaker 46 based on the temperature of itsvoice coil 12. Signals 50 and 52, which are representative of thevoltage and current present at the loudspeaker, are provided as inputsto control element 58. Representative voltage signal 50 is supplied toadmittance function 100, which mimics the electrical admittance ofloudspeaker 46. The output of admittance function 100 is supplied toaveraging function 75, which produces the average level of its input.One embodiment of averaging function 75 is an absolute value functionfollowed by a first-order recursive averaging filter. Representativecurrent signal 52 is supplied to a second averaging function 75. Bothaveraging functions 75 are identical and have averaging time specifiedby coefficient 92, T_(A).

[0091] Still referring to FIG. 11, the average level of the measuredcurrent, I, and the average level of the modeled current, I₀, areprovided to gain calculation element 77, which calculates the additionalgain required for the DC resistance of the voice coil of loudspeaker 46to be reduced to prescribed threshold value 95, $\frac{R_{T}}{R_{0}}.$

[0092] Prescribed threshold value 95 is calculated by Equation (6),where T_(T) is the threshold temperature of the voice coil in Celsius.$\begin{matrix}{\frac{R_{T}}{R_{0}} = {1 + {\alpha \left( {T_{T} - T_{0}} \right)} + {\beta \left( {T_{T} - T_{0}} \right)}^{2}}} & (6)\end{matrix}$

[0093] Gain calculation element 77 calculates its result in Equation(7), where A_(I) and A_(V) are the respective gains of the current andvoltage monitoring circuits described previously. $\begin{matrix}{{gain} = \frac{R_{0}A_{I}I}{R_{T}A_{V}I_{0}}} & (7)\end{matrix}$

[0094] The remainder of control element 58 in FIG. 11 is identical tothat of FIG. 4, with the exception of averaging function 75, which hasbeen described previously.

[0095] While certain exemplary embodiments have been described and shownin the accompanying drawings, it is to be understood that suchembodiments are merely illustrative of and not restrictive on the broadinvention, and that this invention not be limited to the specificconstructions and arrangements shown and described, since various othermodifications may occur to those ordinarily skilled in the art.

What is claimed is:
 1. An apparatus to limit power to a load,comprising: a power source to drive the load using an input signal; avoltage monitor coupled to the power source to detect a voltage suppliedby the power source and to provide a voltage signal representative ofsaid voltage; a current monitor coupled to the power source to detect acurrent supplied by the power source and to provide a current signalrepresentative of said current; the control circuit to receive saidvoltage signal and said current signal, said control circuit to providea value based on said voltage signal and said current signal accordingto one or more control parameters; and, a signal attenuator coupled tothe power source and the control circuit, the signal attenuator to limitsaid input signal based on said value.
 2. The apparatus of claim 1further comprising an amplifier coupled to the power source and theload.
 3. The apparatus of claim 2 wherein the amplifier includes thevoltage monitor and the current monitor.
 4. The apparatus of claim 1wherein the voltage signal is sampled by a first analog-to-digitalconverter to generate a digital voltage signal.
 5. The apparatus ofclaim 4 wherein the current signal is sampled by a secondanalog-to-digital converter to generate a digital current signal.
 6. Theapparatus of claim 5 wherein the control circuit further comprises amultiplier to receive the digital voltage signal and the digital currentsignal, and to calculate an instantaneous power.
 7. The apparatus ofclaim 1 wherein the control circuit implements a linear gain control. 8.The apparatus of claim 1 wherein the control circuit implements anonlinear gain control.
 9. The apparatus of claim 1 wherein the controlcircuit implements a recursive gain control.
 10. The apparatus of claim1 wherein the control circuit implements a non-recursive gain control.11. The apparatus of claim 1 wherein the one or more control parameterscomprises a power averaging time and a power threshold.
 12. Theapparatus of claim 11 wherein the one or more control parameters furtherincludes an attack time and a release time.
 13. The apparatus of claim11 wherein an averaging coefficient (T_(A)) is calculated by the controlcircuit using the power averaging time according to${T_{A} = ^{\frac{- n}{t_{a}f_{S}}}},$

where n is a filter order, t_(a) is the power averaging time in seconds,and f_(S) is a sampling frequency.
 14. The apparatus of claim 12 whereinsaid value is a gain value, and wherein the control circuit calculatesthe gain value using the power threshold expressed as follows:${{gain} = \sqrt{\frac{P_{T}A_{I}A_{V}}{L}}},$

where L is an averaged power level, P_(T) is the power threshold, A_(I)is a corrective factor for the current signal, A_(V) is a correctivefactor for the voltage signal and T_(R) is a release coefficient. 15.The apparatus of claim 14 wherein the release coefficient is calculatedby the control circuit according to,${T_{R} = ^{\frac{- n}{t_{R}f_{S}}}},$

where n is a filter order, t_(R) is the release time, and f_(S) is asampling frequency.
 16. The apparatus of claim 1 wherein the one or morecontrol parameters are selected from a library of control parametersthat is accessible using a graphical user interface.
 17. The apparatusof claim 16 wherein the load is a loudspeaker and the library of controlparameters comprises optimized control parameters for a plurality ofparticular loudspeakers.
 18. The apparatus of claim 17 wherein the oneor more control parameters are selected by a user from the library ofcontrol parameters by selecting from a list of available loudspeakersusing the graphical user interface.
 19. The apparatus of claim 1 whereinthe one or more control parameters are manually provided to the controlcircuit by a user.
 20. The apparatus of claim 1 wherein said one or morecontrol parameters comprises a thermal threshold value.
 21. Theapparatus of claim 20, wherein the thermal threshold value is calculatedaccording to,${\frac{R_{T}}{R_{0}} = {1 + {\alpha \left( {T_{T} - T_{0}} \right)} + {\beta \left( {T_{T} - T_{0}} \right)}^{2}}},$

where α and β are thermal coefficients of resistance, T₀ is a resistanceof said load at ambient temperature and T_(T) is a threshold temperatureof the load.
 22. The apparatus of claim 21, wherein said value is a gainvalue, and wherein the control circuit calculates said gain value usingthe thermal threshold value expressed as follows:${{gain} = \frac{R_{0}A_{I}I}{R_{T}A_{V}I_{0}}},$

where A_(I) is a corrective factor for the current signal, A_(V) is acorrective factor for the voltage signal, I₀ is representative of amodeled current and I is representative of a measured current.
 23. Anapparatus to limit power to a load, comprising: a power source to drivethe load; a monitor coupled to the power source to detect a power levelsupplied by the power source and to provide a power signalrepresentative of said power level; the control circuit to receive saidpower signal and to provide a value based on said power signal accordingto one or more control parameters; and, a signal attenuator coupled tothe power source and the control circuit, the signal attenuator to limitsaid power level based on said value.
 24. The apparatus of claim 23further comprising an amplifier coupled to the power source and theload.
 25. The apparatus of claim 24 wherein the amplifier includes themonitor.
 26. The apparatus of claim 23 wherein the power signal issampled by an analog-to-digital converter to generate a digital powersignal.
 27. The apparatus of claim 26 wherein the control circuitfurther comprises a multiplier to receive the digital power signal, andto calculate an instantaneous power.
 28. The apparatus of claim 23wherein the control circuit implements a linear gain control.
 29. Theapparatus of claim 23 wherein the control circuit implements a nonlineargain control.
 30. The apparatus of claim 23 wherein the control circuitimplements a recursive gain control.
 31. The apparatus of claim 23wherein the control circuit implements a non-recursive gain control. 32.The apparatus of claim 23 wherein the one or more control parametersincludes a power averaging time and a power threshold.
 33. The apparatusof claim 32 wherein the one or more control parameters further includesan attack time and a release time.
 34. The apparatus of claim 32 whereinan averaging coefficient (T_(A)) is calculated by the control circuitusing the power averaging time according to${T_{A} = ^{\frac{- n}{t_{a}f_{S}}}},$

where n is a filter order, t_(a) is the power averaging time in seconds,and f_(S) is a sampling frequency.
 35. The apparatus of claim 33 whereinsaid value is a gain value, and wherein the control circuit calculatesthe gain value using the power threshold according to,${{gain} = \sqrt{\frac{P_{T}A_{I}A_{V}}{L}}},$

where L is an averaged power level, P_(T) is the power threshold, A_(I)is a corrective factor for the current signal, A_(V) is a correctivefactor for the voltage signal and T_(R) is a release coefficient. 36.The apparatus of claim 35 wherein the recovery coefficient is calculatedby the control circuit expressed as follows:${T_{R} = ^{\frac{- n}{t_{R}f_{S}}}},$

where n is a filter order, t_(R) is the release time and f_(S) is asampling frequency.
 37. The apparatus of claim 23 wherein the one ormore control parameters are selected from a library of controlparameters that is accessible using a graphical user interface.
 38. Theapparatus of claim 37 wherein the load is a loudspeaker and the libraryof control parameters contains optimized control parameters for aplurality of particular loudspeakers.
 39. The apparatus of claim 38wherein the one or more control parameters are selected by a user fromthe library of control parameters by selecting from a list of availableloudspeakers using the graphical user interface.
 40. The apparatus ofclaim 23 wherein the one or more control parameters are manuallyprovided to the control circuit by a user.
 41. The apparatus of claim 23wherein said one or more control parameters comprises a thermalthreshold value.
 42. The apparatus of claim 41, wherein the thermalthreshold value is calculated according to,${\frac{R_{T}}{R_{0}} = {1 + {\alpha \left( {T_{T} - T_{0}} \right)} + {\beta \left( {T_{T} - T_{0}} \right)}^{2}}},$

where α and β are thermal coefficients of resistance, T₀ is a resistanceof said load at ambient temperature and T_(T) is a threshold temperatureof the load.
 43. The apparatus of claim 42, wherein said value is a gainvalue, and wherein the control circuit calculates said gain value usingthe thermal threshold value expressed as follows:${{gain} = \frac{R_{0}A_{l}I}{R_{T}A_{V}I_{0}}},$

where A_(I) is a corrective factor for the current signal, A_(V) is acorrective factor for the voltage signal, I₀ is representative of amodeled current and I is representative of a measured current.
 44. Amethod for limiting power to a load comprising: driving the load with aninput signal; providing a voltage signal that is representative of avoltage of the input signal; providing a current signal that isrepresentative of a current of the input signal; calculating a valuebased on said voltage signal and said current signal according to one ormore control parameters; and, limiting the input signal based on thevalue.
 45. The method of claim 44 further comprising amplifying theinput signal before said driving the load with the input signal.
 46. Themethod of claim 44 further comprising generating a digital voltagesignal by sampling the voltage signal with a first analog-to-digitalconverter.
 47. The method of claim 44 further comprising generating adigital current signal by sampling the current signal with a secondanalog-to-digital converter.
 48. The method of claim 47 furthercomprising calculating an instantaneous power by combining the digitalvoltage signal and the digital current signal.
 49. The method of claim44 further comprising implementing linear gain control using a controlcircuit that is used for said calculating the gain value.
 50. The methodof claim 44 further comprising implementing nonlinear gain control usinga control circuit that is used for said calculating the gain value. 51.The method of claim 44 further comprising implementing recursive gaincontrol using a control circuit that is used for said calculating thegain value.
 52. The method of claim 44 further comprising implementingnon-recursive gain control using a control circuit that is used for saidcalculating the gain value.
 53. The method of claim 44 wherein the oneor more control parameters includes a power averaging time and a powerthreshold.
 54. The method of claim 53 wherein the one or more controlparameters further includes an attack time and a release time.
 55. Themethod of claim 53 further comprising calculating an averagingcoefficient (T_(A)) using the power averaging time according to${T_{A} = ^{\frac{- n}{t_{a}f_{S}}}},$

where n is a filter order, t_(a) is the power averaging time in seconds,and f_(S) is a sampling frequency.
 56. The method of claim 54 whereinsaid value is a gain value, the method further comprising calculatingthe gain value using the power threshold according to,${{gain} = \sqrt{\frac{P_{T}A_{I}A_{V}}{L}}},$

where L is an averaged power level, P_(T) is the power threshold, A_(I)is a corrective factor for the current signal, A_(V) is a correctivefactor for the voltage signal and T_(R) is a release coefficient. 57.The method of claim 56 further comprising calculating the releasecoefficient according to, ${T_{R} = ^{\frac{- n}{t_{R}f_{S}}}},$

where n is a filter order, t_(R) is the release time and f_(S) is asampling frequency.
 58. The method of claim 44 further comprisingselecting the one or more control parameters from a library of controlparameters that is accessible using a graphical user interface.
 59. Themethod of claim 58 wherein the load is a loudspeaker and the library ofcontrol parameters comprises optimized control parameters for aplurality of particular loudspeakers.
 60. The method of claim 59 whereinsaid selecting the one or more control parameters comprises selectingthe one or more control parameters from the library of controlparameters by selecting from a list of available loudspeakers using thegraphical user interface.
 61. The method of claim 44 further comprisingmanually selecting the one or more control parameters by a user.
 62. Themethod of claim 44 wherein said one or more control parameters comprisesa thermal threshold value.
 63. The method of claim 62, furthercomprising calculating the thermal threshold value expressed as follows:${\frac{R_{T}}{R_{0}} = {1 + {\alpha \left( {T_{T} - T_{0}} \right)} + {\beta \left( {T_{T} - T_{0}} \right)}^{2}}},$

where α and β are thermal coefficients of resistance, T₀ is a resistanceof said load at ambient temperature and T_(T) is a threshold temperatureof the load.
 64. The method of claim 63, wherein said value is a gainvalue, and wherein the method further comprises calculating said gainvalue using the thermal threshold value according to,${{gain} = \frac{R_{0}A_{l}I}{R_{T}A_{V}I_{0}}},$

where A_(I) is a corrective factor for the current signal, A_(V) is acorrective factor for the voltage signal, I₀ is representative of amodeled current and I is representative of a measured current.
 65. Amethod for limiting power to a load comprising: driving the load with aninput signal from a power source; providing a power signal that isrepresentative of a power level of the input signal; calculating a valuebased on said power signal according to one or more control parameters;and, limiting the input signal based on the value.
 66. The method ofclaim 65 further comprising generating a digital power signal bysampling the power signal with a analog-to-digital converter.
 67. Themethod of claim 65 further comprising calculating an instantaneous powerbased on the digital power signal.
 68. The method of claim 65 whereinthe one or more control parameters includes a power averaging time, apower threshold, an attack time and a release time.
 69. The method ofclaim 68 further comprising calculating an averaging coefficient (T_(A))using the power averaging time according to${T_{A} = ^{\frac{- n}{t_{a}f_{S}}}},$

where n is a filter order, t_(a) is the power averaging time in seconds,and f_(S) is a sampling frequency.
 70. The method of claim 68 whereinthe value is a gain value, and the method further comprises calculatingthe gain value using the power threshold according to,${{gain} = \sqrt{\frac{P_{T}A_{I}A_{V}}{L}}},$

where L is an averaged power level, P_(T) is the power threshold, A_(I)is a corrective factor for the current signal, A_(V) is a correctivefactor for the voltage signal and T_(R) is a release coefficient. 71.The method of claim 70 further comprising calculating the releasecoefficient according to, ${T_{R} = ^{\frac{- n}{t_{R}f_{S}}}},$

where n is a filter order, t_(R) is the release time and f_(S) is asampling frequency.
 72. The method of claim 65 further comprisingselecting the one or more control parameters from a library of controlparameters that is accessible using a graphical user interface, andwherein said load is a loudspeaker and the library of control parameterscontains optimized control parameters for a plurality of particularloudspeakers.
 73. The method of claim 72 wherein said selecting the oneor more control parameters comprises selecting the one or more controlparameters from the library of control parameters by selecting from alist of available loudspeakers using the graphical user interface. 74.The method of claim 65 wherein said one or more control parameterscomprises a thermal threshold value.
 75. The method of claim 74 furthercomprising calculating the thermal threshold value according to,${\frac{R_{T}}{R_{0}} = {1 + {\alpha \left( {T_{T} - T_{0}} \right)} + {\beta \left( {T_{T} - T_{0}} \right)}^{2}}},$

where α and β are thermal coefficients of resistance, T₀ is a resistanceof said load at ambient temperature and T_(T) is a threshold temperatureof the load.
 76. The method of claim 75, wherein said value is a gainvalue, and wherein the method further comprises control circuitcalculates said gain value using the thermal threshold value expressedas follows: ${{gain} = \frac{R_{0}A_{I}I}{R_{T}A_{V}I_{0}}},$

where A_(I) is a corrective factor for the current signal, A_(V) is acorrective factor for the voltage signal, I₀ is representative of amodeled current and I is representative of a measured current.